One useful measure of a portrait (not a requirement but something you can judge) is whether you can show it to someone and they can recognize the person.

I have a print of a friend, which I thought was a great portrait. But over dinner, talking with a friend who is sending her daughter on an upcoming camping trip... My wife mentioned his name. Our friend didn't immediately remember who we were talking about, so I went to get the print.

Showed it to her, and since a baseball cap shaded his face, she couldn't recognize him - even though the portrait reflects his essence TO ME, it failed to help show who we were talking about.

Got out the 4x5 film from the freezer and am going to load up some Grafmatics... Got another shot at portraiture on the camping trip.